1. Technical Field
This invention relates generally to producing machine-readable documents using magnetic toners, and more particularly to controlling the magnetic signal strength of the magnetic images.
2. Background Art
The use of magnetic toner for creating magnetic image character recognition (MICR) such as used for bank checks is well known. U.S. Pat. No. 4,563,086 to Knapp et al. discloses such a printer wherein the intensity of the magnetic field generated by the toner image is detected and used to regulate various process control parameters of the printer; including photoconductor charging, exposure, development bias, toner concentration, and transfer voltage.
Often, users of MICR printers and copiers find that it is necessary or desirable to adjust the magnetic signal strength of the image. The need for this ability arises from a number of variables such as environmental conditions, paper type, the age and condition of the photoconductor, and the user's preference. For example, some characters are formed with more toner than are other characters, and a character chain containing a greater percentage of the former characters would exhibit a greater overall signal strength than a chain containing a lesser percentage of the former characters. Accordingly, a user might wish to decrease the signal strength of each character when reproducing chains containing a greater percentage of the former characters. Another example of when a user might wish to adjust the MICR signal strength is when the character orientation changes between landscape and portrait mode. The sharpness of the leading edge of a character is affected by its orientation relative to the direction of movement of the edge through the development station, and a sharp leading edge tends to produce a greater MICR signal strength.
Because of their complexity, prior art processes for adjusting MICR signal strength often require highly skilled workers, such as field engineers or technical representative service persons.
When the magnetic signal strength is adjusted by means of regulation of the concentration of magnetic toner particles in the development mixture, as is done in said U.S. Pat. No. 4,563,086, added toner particles must be thoroughly mixed into the developer before a print is produced, and toner can be removed from the mixture only by producing many wasted prints. This slows the adjustment process and makes it difficult to effect. Over adjustments are cumbersome to recover from.